The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device. In particular, the present invention relates to a semiconductor device, which has a feature in the method of forming a silicon oxide film, and to a method of manufacturing the semiconductor device thereof.
An NAND type EEPROM, which is low in manufacturing cost and excellent in reliability as well as in high speed programming property is now attracting attentions as an alternative device to a magnetic memory. This NAND type EEPROM is provided with two different kinds of electrodes, i.e., an electrode (a floating gate electrode) for storing charges and an electrode (a control gate electrode) for forming an electric field for charging or discharging the charges to/from floating gate electrode.
According to the semiconductor memory of this kind, the electrically programming and erasing are performed by applying a high voltage to the control gate electrode so as to charge or discharge electrons to/from the floating gate electrode from the substrate through the gate oxide film.
Since the transfer of electrons between the substrate and the floating gate electrode is performed by making use of the Fowler-Nordheim tunnel mechanism, the gate oxide film interposed between the substrate and the floating gate electrode is called a tunnel oxide film.
One of the important features required for this tunnel oxide film is to make the total quantity of passing electrons (Qbd) that results in the dielectric breakdown of the oxide film larger than that of the ordinary MOS transistor. Because, since the memory is employed by taking advantage of the transfer of electrons by applying a high electric field to the tunnel oxide film, if the value of Qbd is small, the reliability of the oxide film would be deteriorated or the life thereof would be shortened. At the same time, it is also important to minimize so-called stress-induced leak current, which is a phenomenon of increasing a leak current at low electric field due to the application of high electric field to the tunnel oxide film.
A thermal oxidation method, i.e., a method of heating a silicon substrate while exposing it to a dry oxygen atmosphere, has been conventionally employed as a typical method of forming a tunnel oxide film. The quality of the thermal oxide film to be obtained from this thermal oxidation method is known to vary depending on the oxidation process conditions such as the oxide film-forming temperature (oxidation temperature), the heat treatment (annealing) temperature in a non-oxidative gas atmosphere, the annealing time and the rate of raising or lowering temperature. Therefore, many attempts have been tried to improve the dielectric breakdown resistance of the oxide film by adjusting these oxidation process conditions. However, no one has succeeded up to date to obtain a thermal oxide film, which meets the dielectric breakdown resistance, which may be required in future.
On the other hand, with viewpoints of saving the manufacturing cost of a semiconductor device and of simplifying a semiconductor device manufacturing apparatus, it is now desired to lower the oxidation temperature of the oxide film. According to the conventional thermal oxidation method, there has been developed for obtaining a required oxidation rate a method of raising temperature of substrate at a high speed up to a high temperature (e.g., 1,000.degree. C.). However, this conventional method is accompanied with problems that an apparatus capable of withstanding such a high temperature is high in manufacturing cost, and that since the rate of raising or lowering the substrate temperature is very high, the substrate is imposed by a large thermal stress.
As for the method of forming an oxide film, which meets the aforementioned demands for improving the dielectric breakdown resistance of the tunnel oxide film and for lowering the oxidation temperature, there is known an oxidation method employing an active oxygen species such as oxygen radicals and ozone.
The oxidation method employing oxygen radicals has been developed up to date with a view to lowering the oxidation temperature. For example, according a publication, Applied Physics, Kimura et al., Vol. 56, pp. 64-69 (1987), it is reported that an oxide film exhibiting the same degree of dielectric breakdown resistance as that of a thermal oxide film can be obtained by performing the oxidation in an oxygen radical atmosphere at a temperature of 580.degree. C.
Further, with a view to realize a sufficient oxide film-forming rate and to obtain a silicon substrate-oxide film interface having a property equivalent to a thermal oxide film, there has been reported a method wherein a thin oxide film is formed at first on the surface of a silicon substrate in an oxygen radical atmosphere and then additional oxide film is deposited on the aforementioned thin oxide film by means of a chemical gas phase deposition (CVD) method (Jpn. J. Appl. Phys., Vol. 31, G. Lucovsky, Y. Ma, T. Yasuda, C. Silvestre, J. R. Hauser, pp. 4387-4395 (1992)).
However, an oxygen radical oxide film having a more excellent dielectric breakdown resistance than that of a thermal oxide film has not been reported as yet. The reason for this may be ascribed to the fact that the oxygen radical atmosphere employed up to date for oxidation is formed of a mixture of activated species excited in various states.
As for the means for generating oxygen radicals, a microwave discharge, a parallel plate plasma discharge, or an electron cyclotron resonance plasma discharge in a low pressure oxygen gas atmosphere has been employed up to date. As an alternative means for these discharges, a method of irradiating an excitation light to an oxidative gas has been also proposed.
However, in any of these methods, it is not intended to selectively generate an optimum activated species in the oxidation step, i.e., various kinds of activated species (for instance, oxygen atoms/molecules in various excitation states, positive/negative ions of oxygen atom and positive/negative ions of oxygen molecules) are caused to be generated simultaneously in the atmosphere.
There are inappropriate activated species in the oxidation step, which may react with an activated species, which are optimum for the oxidation, or adsorb on the surface of oxide film. These reaction and adsorption by the inappropriate activated species may hinder the supply of activated species, which are optimum for oxidation to an oxidation film, or become causes for a deterioration of dielectric breakdown resistance of an oxide film being formed or for a slowdown of oxidation rate.
With regard to the oxidation method employing ozone, the lowering of the oxidation temperature and at the same time, the improvement of the dielectric breakdown resistance of oxide film have been reported (for instance, Takasaki, a textbook for The 42nd Semiconductor Professional Meeting (1995)).
As for the cause for the improvement of dielectric breakdown resistance in the oxidation method employing ozone, the following phenomena may be accounted therefor. Namely, when ozone molecules reach the surface of an oxide film facing an oxidation atmosphere, active oxygen radicals are fed into the oxide film and allowed to react with lattice defects which may become a cause for a trap-site of electric charge, and hence the trap-site of electric charge is entrapped in an SiO.sub.2 mesh structure, thus eliminating the trap-site of electric charge.
However, the ozone molecules may become oxygen atoms or oxygen ions taking various electron excitation states, according to a difference in microscopic structure of the surface of oxide film to be fed to the oxide film through the surface of oxide film.
According to the conventional ozone oxidation method, activated species which are optimum for the oxidation and activated species which are not optimum for the oxidation are all fed to the surface of the substrate. Therefore, the conventional ozone oxidation method is not preferable because of the same reasons as explained with reference to the aforementioned oxygen radical oxidation method.
Meantime, for realizing a semiconductor device which is excellent in reliability and capable of exhibiting a high-speed operation property, it is also imperative to improve not only the reliability of insulating layers and wiring layers both constituting a semiconductor device, but also the flatness of interfaces among the insulating layer, wiring layer and semiconductor body.
When a hole or trench is formed selectively on the surface of semiconductor substrate by making use of a reactive ion etching (RIE) method for instance for forming an element isolation region or a capacitor of large electrostatic capacity, the surface of semiconductor substrate including the side walls and bottom surface of the hole or trench is damaged by the etching species such as ions and plasma, thus generating so-called RIE damage. In order to cope with this problem, a countermeasure has been conventionally taken, wherein the surface of semiconductor substrate is subjected to a thermal oxidation in a dry oxygen atmosphere until an oxide film having a thickness of about 50 nm or more is formed on the surface of semiconductor substrate and then this thermal oxide film is removed thereby removing the layer of RIE damage and at the same time assuring the reliability of a gate oxide film or embedded insulating layer to be subsequently formed on the surface of semiconductor substrate.
However, if a thermal oxidation is adopted for the oxidation (buffer oxidation) of the semiconductor substrate for removing the layer of RIE damage, the interface between the substrate and the thermal oxide film cannot be sufficiently flattened, so that a prominent roughness would be left on the surface of the substrate after the thermal oxide film is removed. Accordingly, if a very thin film gate oxide film is formed through oxidation on this rough surface of the substrate, or if an element isolation or a trench capacitor is formed by embedding such a rough-surfaced hole or trench with an insulating layer, a dielectric breakdown would be likely to be generated due to an interface roughness between the substrate and the insulating layer, or a deterioration of drain current response speed would be caused to generate at the gate oxide film. Furthermore, since the depth of the RIE-damaged layer is at most about 10 nm or less, the buffer oxidation of 50 nm or more in depth that is to be performed to the side wall of the hole or trench would lead to a redundant increase in element area, thus hindering the miniaturization of semiconductor element.
Additionally, it has been conventionally performed, in the step of making an electric connection between a semiconductor and an overlying wiring layer, to selectively form a contact hole in an insulating layer interposed between the semiconductor and the overlying wiring layer so as to expose the semiconductor at the bottom of the contact hole and then to fill the contact hole with a silicide layer or a wiring layer without flattening the exposed bottom surface of the contact hole in advance.
However, if the roughness of the exposed surface of the semiconductor at the bottom of the contact hole is large, the uniform growth of the silicide layer or of the embedded wiring layer would be hindered, thus introducing an electric contact failure between the wiring layer and the semiconductor, or a deterioration in reliability of the wiring layer. Further, in the case where an impurity-doped layer is formed on the surface of the semiconductor, a phenomenon where an electric charge flows passing through this impurity-doped layer may be induced. According to the conventional buffer thermal oxidation, a temperature of about 1,000.degree. C. is required as an oxidation temperature, so that, because of the possibility of a thermal deformation of the region around the contact hole that might be caused by the buffer thermal oxidation, it has been considered very difficult to perform the buffer oxidation of the bottom surface of contact hole.
Meanwhile, as a ultra-flattening method of the surface of substrate where the flatness is discussed at the atomic level, there are proposed a method of heating a natural oxide-deposited substrate at a temperature of 1,000.degree. C. in a ultra-high vacuum atmosphere (flushing, for example, M. Niwa, T. Kouzaki, K. Okada, M. Udagawa, and R. Sinclair, Japanese Journal of Applied Physics, Vol. 33, pp. 388 (1994)), and a molecular beam epitaxial growth method (for example, A. Ourmazd, D. W. Taylor, J. A. Rentschler, and J. Bevk, Physical Review Letter, Vol. 59, pp. 213 (1987)). However, these methods are not feasible because of the treating temperature, the treating time or the cost of apparatus, and, in addition to that, may introduce a contamination of the insulating layer with a metal constituting the inner wall of the apparatus, thus resulting in the dielectric breakdown of the insulating layer.
As mentioned above, since the conventional method of forming an oxide film employing oxygen radical or ozone is not intended to employ an activated species which is optimum for the oxidation, the dielectric breakdown resistance of the oxide film is poor and the oxide film-forming rate is also low as compared wiith a method where an activated species which is optimum for the oxidation is employed.
As explained above, when a thermal oxidation in a dry oxygen atmosphere is employed in the buffer oxidation of a semiconductor substrate, it is impossible to assure a sufficiently flattened surface of the semiconductor substrate after the peeling of the buffer oxide film, so that the reliability of not only the gate oxide film but also the embedded insulating layer which are to be subsequently formed on the surface of the substrate would be deteriorated. Further, since the exposed surface of semiconductor substrate, which constitutes the bottom surface of contact hole, is not sufficiently flattened according to the prior art, the reliability and property of the semiconductor device are caused to deteriorate.